Interactions between bacterial pathogens and host innate immunity often determine the outcome of an infection. Toll-like receptors (TLRs) induce antimicrobial mechanisms, but many bacteria have evolved virulence strategies that allow them to evade aspects of this host response. The importance of TLRs for immunity to Salmonella typhimurium has been demonstrated using cells or mice deficient in TLRs or TLR signaling components. A caveat of these studies, however, is that they are typically performed with highly susceptible inbred mouse strains due to a non-functional allele of the Nramp-1 gene. We hypothesize that the extreme susceptibility of Nramp-1 mutant mice may mask host-pathogen interactions between S. typhimurium and the host innate immune system. Accordingly, we have generated and analyzed TLR-deficient mice with functional Nramp-1. These analyses have resulted in a surprising finding: TLR function is required for S. typhimurium survival and replication in macrophages and for virulence in mice. In macrophages lacking TLR function, S. typhimurium fails to induce virulence genes required for intracellular survival and replication. In Aim 1, we will identify the TLR-dependent signal used by S. typhimurium to coordinate virulence gene induction. In Aim 2, we will examine in which cell types TLR signaling is required for S. typhimurium virulence in vivo. In Aim 3, we will switch to the host side of the host-pathogen interaction and examine why TLR4 plays such a dominant role in the host response to S. typhimurium infection.